1. Field of the Invention
The present invention relates to a vertical cavity surface emitting laser device.
2. Description of the Related Art
It is considered that in future, a large number of surface emitting laser (VCSEL) devices that emit a laser light in a wavelength band of 1000 nm to 2500 nm with an operating speed equal to or higher than 10 Gbps will be required in the fields of optical interconnection and optical communication.
As a conventional surface emitting laser device, a vertical cavity surface emitting laser device in which a distributed Bragg reflector (DBR) laser is built by forming an active layer between an upper and a lower multilayer reflectors made of semiconductor and forming a current confinement layer that confines the current path to improve current injection efficiency has been disclosed (see Japanese Patent Application Laid-Open No. 2005-252032). The current confinement layer is formed by selectively oxidizing a layer made of AlAs, and includes a current confining portion in a circular shape made of AlAs disposed in the center and a selectively oxidized portion made of oxidized aluminum disposed around the current confining portion.
Meanwhile, a surface emitting laser device for a long wavelength band formed with an active layer in a multiple quantum well structure made of GaInNAs-based materials on a GaAs substrate is also attracting attention. For example, a GaInNAs-based active layer for the wavelength band of 1.3 μm to 1.55 μm is a so-called high strain system with an amount of compressive strain of the quantum well layer equal to or more than 2%. Therefore, in general, from a viewpoint of the critical film thickness of the quantum well layer, the thickness of a barrier layer is 20 nm to 25 nm, and the number of quantum wells is two to three. The active layer, for example, is formed by alternately growing a quantum well layer of Ga0.63In0.37N0.01As0.99 with a thickness of 7.3 nm, and a barrier layer made of GaAs with a thickness of 20 nm to 25 nm, and the number of the quantum well is three.
With a surface emitting laser device for a long wavelength band that adopts an active layer in a quantum dot structure made of InAs or InGaAs, a barrier layer has the same thickness as the above, because the quantum dot is highly strained.
Accordingly, the conventional surface emitting laser device with the highly strained active layer has a high threshold current and a low optical output, thereby operating in a low speed without increasing the relaxation frequency.
By making the barrier layer thin and by disposing each quantum well layer close to a position where the amplitude of an optical standing wave generated in an optical resonator is large, a modal gain can be increased because an optical confinement factor to the quantum well layer will be increased and the threshold current can be lowered (see A. W. Jackson, et al., “OC-48 capable InGaAsN vertical cavity lasers”, Electronics Letters, vol. 37 No. 6, pp. 355-356 (2001)).
However, with the highly strained active layer, there is a problem that a dislocation such as a misfit dislocation tends to occur to the quantum well layer, when the thickness of the barrier layer is made thin, thereby degrading the reliability of the device. As a result, the thickness of the barrier layer had to be thick, and the modal gain could not be increased.